Phase distortion correction



Oct. 13, 1942.

A. DECINO PHASE DISTORTION CORRECTION Filed Jan. 14, 1941 SIGNALSSIGNALS PHA SE INVENTOR A. DEC/NO AZTORNEV Patented Oct. 13, 1 942 PHASEDISTORTION CORRECTION Alfred Decino, Elbel'on Park, N. .1., assignor to,Bell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application January 14., 1941, serial No.374,337/

3 Claims. (01. 179-47115) The present invention relates to wavetransmission for signaling or other purposes, using modulated waves.More specifically, the invention relates to the reduction of effects dueto unwanted phase modulation in an amplitude modulated system.

Undesired phase modulation may be introduced in an amplitude modulatedwave in a variety of ways among which are: improper neutralization of abridge-neutralized modulator stage, improper tuning of the outputcircuits and, at ultra-high frequencies, the eifects of variations inelectron transit time from the fact that the plate voltage is changingthrough the modulation cycle.

The phase modulation produced at the transmitter shows up as distortionin the receiver output when the receiver gain or phase characteristicversus frequency is unsymmetrical about the mid-band frequency or theband width is narrow. This is likely to be the case where the total bandwidth is limited to the minimum amount for passing the multiplexchannels by means of filters with sharp cut-off at the edges of thetransmission band. In such instances the phase modulation is detected bythe amplitude detector and appears as added distortion in the desiredsignal in the form of cross-talk or cross-modulation in a multiplexsystem due to the products that fall into the other channels. It ishighly desirable, therefore, to suppress the phase modulation as well asto improve the over-all transmission characteristic.

It is common in radio transmitters to employ over-all or envelopenegative feedback to reduce the total distortion occurring anywhere inthe transmitter. This is done by detecting some of the output wave torecover the speech together with distortion and feeding the detectedcomponents into the speech input circuit in reverse phase. Phasemodulation is not reduced by this procedure since the phase modulationis not fed back, at least in any way that is capable of producingcompensation for the phase modulation.

In accordance with the present invention, phase modulation is detectedat the transmitter and a feedback action is produced which results inreducing the phase modulation. The action is similar in principle tonegative feedback in re ducing amplitude distortion but the manner inwhich the negative feedback is applied to reduce the phase modulation isbelieved to be novel.

Phase modulation of the carrier may be caused cycle power used to heatthe cathodes-or by both. The invention' is efiective, in'its differentforms of embodiment-tribe described, to reduce one or the other typeorboth of these" types :0!- modulation. 1 f

The nature and objects of the invention will be more fully understood mthe following detailed descriptionin connection with the attacheddrawing in which: I I

. Fig. 1 is a schematic circuit diagram of one form in whichphasemodulation of the carrier is suppressed; I

Fig. 2 is a vector diagram to be referred to in connection with Fig. 1;

Fig. 3 shows a'modification in the phase compensating circuit of Fig. 1;and

Fig. 4 shows another form of the invention for reducing the eflects ofphase modulation of the carrier.

Referring to Fig. 1, the oscillator I may be of any suitable type suchas a crystal controlled oscillator designed to produce a wave at acon--' venient frequency level, the frequency of which may be stepped upby one ormore stages offharmonic generation, two being represented at 2and 3. The carrier wave is then passed through a phase controllingcircuit 4, to be described, to the input of the modulating circuit 5which may be of any suitable type such as push-pull. It may comprisemore than one stage and may include amplifiers as needed. The speech orother signal is introduced into the plate circuit of the modulatorthrough speech amplifier 6. .The output wav consisting preferably ofboth side-bands and the unmodulated carrier component is radiated fromantenna 1. It will beunderstood' .on the drawing. While negativefeedback for amplitude distortion reduction is not shown it may be usedin known manner.

A portion of the "output wave is picked up in i the input of radiofrequency amplifier 8 through by the speech or signal or by noise suchas the coupling 9.' This amplifier is'shown provided' with a, detectorl0 and automatic volume control ll set for sufliciently fast action tocounteract-the speech variations'in'the carrier and toproduc'e in thecircuit i2 a substantially con stant amplitude wave. Iii-placeof'elements Ill and II a limiter could be-used in the circuit I2 or gaincontrol and limiter action could be used the length of the resultantdecreases, its

together. It the modulation is substantially 100 Detector I3 is suppliedalso with unmodulated rrier waves obtained from harmonic generator l: byway of coil l1 and (it needed) phase shifter l8. The unmodulated carriercomponents applied to the'detector from 14 and ll are ad usted to beequal in amplitude and to be normally in phase quadrature as indicatedby the vectors v1! and V11 0! Fig. 2. The two vectors produce a vector25 the length 0! which represents the normal or zero adjustment when thetwo component vectors are in 90-degree relation. It the angle betweenthe component vectors decreases, the resultant 20 increases in lengthand if the angle increases arrow having a locus represented by thedotted semicircle, assuming vector V12 as stationary. The output of therectifier detector l3 represents, therefore, a current whose magnitudevaries as the phase angle between the two carrier frequency voltagesimpressed on the detector varies. resultant current flows throughresistor 2| which is by-passed for the carrier by capacity 22. A biascontrol lead 23 is connected to a slider on resistor 2| for deriving acontrol voltage therefrom for application to the grid 0! reactance tube24. A filter 25 may be included in the bias control lead to pass certainfrequencies only if desired as will be described later on.

The change in rectified current obtained from detector i3 is used tovary the bias on the grid of control tube 24 whose plate circuitimpedance is thereby adjusted so as to produce a change in its shuntreactance. This changing impedance is used to control the phase or theradio frequency driving voltage to the modulator 5 so asto compensatefor the phase modulation occurring in the radio transmitter. In theembodiment shown in Fig. 1 the control tube 24 varies the reactance oithe series resonant circuit 4.

The vector V11 of Fig. 2 is unaffected by amplitude modulation and maybe assumed constant in length and phase. If the vector V1: is also oiconstant length notwithstanding the amplitude modulation of the carrierso that only the phase angle between the two vectors is effective inchanging the length of vector 20, then the compensation of the phasemodulation can be accomplished in high degree. If, however, the vectorV1: varies in length as well as in its angular position with respect tovector V17, these length variations representing amplitude modulationshould not be allowed to be translated mto phase variations in thecarrier since they would produce spurious effects. If the amplitudemodulation present in voltage V1: is confined to a particular frequencyrange, such range may be suppressed by fllter 25 and the fluctuations ofsuch range are prevented .from' affecting the control tube 24. Forexample, let it be supposed that phase modulation of -the carrier atpower frequencies below the speech range is objectionably large and thatthe coupling l4, l5 impresses on the detector l3 some speech amplitudemodulation which if allowed to affect the control tube would produceobjectionable phase modulation.

In such case the filter 25 serves to suppress the speech frequencyfluctuations but the power frequency variations such as (SO-cyclevariations are allowed to pass on to control tube 24 and eilect thenecessary phase compensation.

Fig. 3 illustrates another manner of connecting the control tube 24 tothe carrier supply path leading to the modulator 5. The carrier outputof the harmonic generator 3 feeds into a carrier amplifier through tunedcoupling 33, the output of the amplifier including tuned coupling 34.Control tube 24 has its plate impedance shunted across the tunedcoupling 34 so that variations in the plate impedance of tube 24 producecorresponding variations in the reactance in shunt of coupling 34 andconsequent variations in the phase of the carrier transmitted to themodulator. Still other types or control tube coupling are possible.

Referring to Fig. 4, the oscillator I, harmonic generator 2, amplifier30 with its input coupling 33 and output coupling 34, modulator 5 andantenna I may all be as previously described, the harmonic generator 2comprising, if desired, a plurality of stages of frequencymultiplication to arrive at the desired carrier frequency.

It was stated above that phase modulation in the transmitted wave isparticularly objectionable in multiplex systems in which the tunedreceivers may have unsymmetrical phase characteristics giving rise toamplitude distortion in the detected wave. It is assumed in Fig. 4 thatthe detector 35 with its tuned selector circuit 36 has an unsymmetricalphase characteristic so as to detect the phase modulation for controlpurposes. As a result the detected current in resistor 31 will becomposed of detected amplitude modulation (the signal and detected phasemodulation (distortion) resulting from the unsymmetrical phasecharacteristic of the detector circuit. A voltage of proper value ispicked ofi the resistor 31 by the slider and conveyed over bias controllead 23 tothe grid of control tube 24. Inserted in lead 23 is the speechcoupling 39 leading from the output of speech amplifier 6 and derived,for example, from potentiometer resistance 4|. The purpose of the speechcoupling 39 is to apply a cancelling speech voltage to lead 23 adjustedin magnitude and phase so as exactly to balance out that part of thedetected voltage in lead 23 that isdue to the amplitude modulation butto leave as a residue that part of the detected voltage that is due tothe phase modulation. For this purpose a phase shifter 40 may benecessary between resistor 4| and the coupling 39. The adjustmentsshould be such that only the distortion due to phase modulation isapplied to the grid of control tube 24, and the magnitude and phase ofthis distortion so applied should be of the proper values to produce theappropriate compensating phase premodulation of the carrier passing fromthe driver stage 30 to modulator 5. Phase shift circuit 38 is providedfor producing the necessary phase relation in the control voltage.

If desired, a negative feedback path may be provided from a suitableoutput point such as winding 42 coupled to the modulator output, throughdetector 43 to the speech input circuit on the input side of amplifier6. This path provides negative envelope feedback for reducing amplitudedistortion in the output wave, in known manner. It is, of course,desirable to keep the amplitude distortion as low as possible in thetransmitted wave in the interest of high quality transmission and, also,in connection with the invention, so that the residue of speech currentin lead 23 remaining after comparison with some of the input speech incoupling 39 shall consist so far as possible of distortion due solely tophase modulation.

What is claimed is:

V 1. In a system for modulating the amplitude of carrier waves by aninput signal with incidental phase modulation of said carrier waves,means to detect the amplitude modulation and the phase modulation, meanscomprising a differential circuit for combining a portion of the inputsignal with the output of said detecting means to separate thecomponents resulting from phase modulation from the components resultingfrom amplitude modulation, and means to produce compensating phasemodulation of the carrier in accordance with the former components.

2. In a transmitting system for signal amplitude modulated carrier wavespossessing gain from input to output, a source of input signal signalwith the output of said detecting means to separate the components dueto amplitude modulation from the components due to phase modulation, andmeans to feed back the latter components in gain reducing sense tocounteract the incidental phase modulation.

3. In a radio transmitter, a source of input signal wave, a source ofhigh frequency waves, a radio transmitter including means to modulatethe amplitude of said high frequency waves by said signal wave fortransmission, said radio transmitter producing incidental modulation-ofthe phase of said high frequency waves, means to detect some of thephase modulation of the transmitted waves,'said last means alsoproducing incidental detection of the amplitude modulation thereof,means to eliminate such incidental detected amplitude modulationcomprising a differential coupling for combining a portion of the inputsignal wave with the output of said detecting means to cancel the signalcomponent resulting from detection of the amplitude modulation, therebyleaving a residue of signal resulting substantially solely fromdetection of the phase modulation, and means to modulate the phase ofthe high frequency wave by said residue, in reverse sense, on the inputside of said am-' plitude modulating means, to reduce the amount of saidincidental phase modulation in the transmitted waves.

ALFRED DECINO.

